Thermally insulated phase change material cells

ABSTRACT

Memory cell structures for phase change memory. An example memory cell structure comprising includes a bottom electrode comprised of electrically conducting material, and phase change material disposed above the bottom electrode. A layer of thermally insulating material is disposed, at least partially, between the bottom electrode and the phase change material. The thermally insulating material is comprised of Tantalum Oxide. A top electrode is comprised of electrically conducting material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. §120 to U.S. patent application Ser. No. 13/364,153 filed on Feb.1, 2012, which is a divisional of and claims priority under 35 U.S.C.§120 to U.S. patent application Ser. No. 12/497,596 filed on Jul. 3,2009, the entire text of which is specifically incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to forming memory cell structures forphase change memory.

2. Description of Background

There are two major groups in computer memory: non-volatile memory andvolatile memory. Constant input of energy in order to retain informationis not necessary in non-volatile memory but is required in the volatilememory. Examples of non-volatile memory devices are Read Only Memory(ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric RandomAccess Memory, Magnetic Random Access Memory (MRAM), and Phase ChangeMemory (PCM). In general, non-volatile memory devices are memory devicesin which the state of the memory elements can be retained for days todecades without power consumption. On the other hand, volatile memorydevices require constant or very frequent energizing to maintain thestate of the memory element. Examples of volatile memory devices includeDynamic Random Access Memory (DRAM) and Static Random Access Memory(SRAM).

The present invention is directed to phase change memory. In phasechange memory, information is stored in materials that can bemanipulated into different phases. Each of these phases exhibitdifferent electrical properties which can be used for storinginformation. The amorphous and crystalline phases are typically twophases used for bit storage (1's and 0's) since they have detectabledifferences in electrical resistance. Specifically, the amorphous phasehas a higher resistance than the crystalline phase.

Chalcogenides are a group of materials commonly utilized as phase changematerial. This group of materials contain a chalcogen (Periodic TableGroup 16/VIA) and another element. Selenium (Se) and tellurium (Te) arethe two most common elements in the group used to produce a chalcogenidesemiconductor when creating a phase change memory cell. An example ofthis would be Ge2Sb2Te5 (GST), SbTe, and In2Se3.

Altering the phase change material's state requires heating the materialto a melting point and then cooling the material to one of the possiblestates. A current passed through the phase change material creates ohmicheating and causes the phase change material to melt. Melting andgradually cooling down the phase change material allows time for thephase change material to form the crystalline state. Melting andabruptly cooling the phase change material quenches the phase changematerial into the amorphous state.

SUMMARY OF THE INVENTION

One aspect of the invention is a memory cell structure. The memory cellstructure includes a bottom electrode comprised of electricallyconducting material and phase change material disposed above the bottomelectrode. A layer of thermally insulating material is disposed, atleast partially, between the bottom electrode and the phase changematerial. The thermally insulating material is comprised of TantalumOxide. A top electrode is comprised of electrically conducting material.

Another aspect of the invention is another memory cell structure. Thememory cell structure includes a bottom electrode comprised ofelectrically conducting material and phase change material disposedabove the bottom electrode. A layer of thermally insulating material isin physical contact with both the bottom electrode and the phase changematerial. The layer of thermally insulating material is disposed, atleast partially, between the bottom electrode and the phase changematerial. A top electrode is comprised of electrically conductingmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 illustrates a substrate, a bottom electrode, and a dielectriclayer with a pore.

FIG. 2 illustrates thermally insulating layer deposition.

FIG. 3 illustrates phase change memory deposition and top electrodeformation.

FIG. 4 illustrates an alternate embodiment of the invention.

FIG. 5 illustrates an alternate embodiment of the invention with a poreelectrode.

FIG. 6 illustrates a substrate, a bottom electrode, and a pillar.

FIG. 7 illustrates thermally insulating layer deposition deposition.

FIG. 8 illustrates dielectric layer deposition.

FIG. 9 illustrates an alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described with reference to embodiments of theinvention. Throughout the description of the invention reference is madeto FIGS. 1-9.

In typical phase change memory configuration, the phase change materialis used to store data bits. An example of such a phase change materialis Germanium-Antimony-Tellurium (GST). The phase change material may beprogrammed to one of at least two states: a crystalline state or anamorphous state (or somewhere in between). The crystalline state mayrepresent a stored “0” value and the amorphous state may represent astored “1” value. In the crystalline state, the phase change materialexhibits a relatively low resistance. On the other hand, in theamorphous state, the phase change material has a relatively highresistance.

In the phase change memory configuration, altering the phase changematerial's state requires heating the material to a melting point andthen cooling the material to one of the possible states. A currentpassed through the phase change material creates ohmic heating andcauses the phase change material to melt. Melting and gradually coolingdown the phase change material allows time for the phase change materialto form the crystalline state. Melting and abruptly cooling the phasechange material quenches the phase change material into the amorphousstate.

As described below, an aspect of the present invention is a method forforming a memory cell structure for phase change memory. The methodincludes forming a bottom electrode within a substrate. The method alsoincludes depositing a dielectric layer above the bottom electrode andforming a pore within a dielectric layer substantially above the bottomelectrode. The method includes depositing a thermally insulatingmaterial along the bottom of the pore and along at least one sidewall ofthe pore. The method also includes forming phase change material abovethe bottom electrode.

The thermally insulating material isolates the heat generated by ohmicheating to the volume contained within the pore. The thermallyinsulating material reduces heat dissipating to the surroundingdielectric material, thus, reducing the energy required to melt thephase change material.

FIG. 1 illustrates the substrate 102, the bottom electrode 104 comprisedof electrode material, the dielectric layer 106 comprised of dielectricmaterial, and the pore 108. The bottom electrode 104 is formed withinthe substrate 102. Those skilled in the art will recognize that avariety of processes may be utilized to form the bottom electrode 104within the substrate 102, such as shallow trench isolation (STI) andmetal sputter deposition. The bottom electrode is comprised of electrodematerial that is electrically conducting, such as but not limited toTungsten (W) or Titanium Nitride (TiN). The dielectric layer 106 is thendeposited above the substrate 102 and bottom electrode 104. A variety ofprocesses may be utilized to deposit the dielectric layer 106, such as,chemical vapor deposition (CVD) or plasma enhanced chemical vapordeposition (PECVD). In one particular embodiment of the invention thedielectric layer 106 is comprised of Silicon Dioxide (SiO₂).

The pore 108 is formed within the dielectric layer 106 and substantiallyover the center of the bottom electrode 104, as shown. A process such asshallow trench isolation may be utilized in forming the pore 108 withinthe dielectric layer 106.

Now turning to FIG. 2, the thermally insulating material 202 isdeposited along the bottom of the pore 108 and along at least onesidewall of the pore 108. Again, those skilled in the art will recognizethat a variety of processes may be utilized in depositing the thermallyinsulating material 202, such as chemical vapor deposition. In oneembodiment of the invention, the thermally insulating material 202 isTantalum Oxide (Ta₂O₅). The thermally insulating material 202 helpsisolate the heat generated by ohmic heating to the volume containedwithin the pore 108.

Shown in FIG. 3 is the deposition of the phase change material 302 overthe bottom electrode 104 and into the pore and within the volume of thethermally insulating layer 202. Those skilled in the art will recognizethat a variety processes may be utilized for phase change material 302deposition, such as chemical vapor deposition. In one particularembodiment of the invention the phase change material 302 isGermanium-Antimony-Tellurium (GST). A top electrode 304 is then formedabove the phase change material 302. Again, a variety of processes maybe utilized for forming the top electrode 304, such as metal sputterdeposition. In one particular embodiment of the invention, the topelectrode 304 is comprised of Titanium Nitride. In one embodiment, thefilm of thermally insulating material allows electrons to tunnel fromthe bottom electrode to the phase change material during operation ofthe memory cell.

FIG. 4 illustrates an alternate embodiment of the invention. In thisparticular embodiment, the thermally insulating material 202 is removedfrom the bottom of the pore and the top surface of the dielectric layer106. The removal occurs prior to the deposition of the phase changematerial 302. Those skilled in the art will recognize that a variety ofprocesses may be utilized to remove the thermally insulating material202 from the bottom of the pore, such as a directional reactive-ion etch(RIE).

FIG. 5 illustrates another embodiment of the invention. In thisparticular embodiment, a pore electrode 502 is formed in the pore. Thepore electrode 502 is comprised of an electrically conducting material.A variety of processes may be utilized in forming the pore electrode502, such as metal sputter deposition. In one particular embodiment, thepore electrode 502 is comprised of Titanium Nitride. Once the poreelectrode 502 is created, the phase change material 302 and the topelectrode 304 are deposited over the pore electrode 502, as describedabove.

Now turning to FIG. 6, yet another embodiment of the invention is shown.In this particular embodiment, the bottom electrode 104 is formed withinthe substrate 102 and a pillar 602 is formed substantially over thecenter of the bottom electrode 104. The pillar 602 includes phase changematerial 704 above the bottom electrode 104 and a top electrode 606above the phase change material 604. The top electrode 606 is comprisedof electrically conducting material. In one embodiment of the invention,the top electrode 606 is comprised of Titanium Nitride.

A variety of processes may be utilized in forming the pillar 602. Anexample of such processes begins with depositing the phase changematerial 604 and the electrode material above the bottom electrode 104.A photolithographic mask is then used to protect the areas above thecenter of the bottom electrode 104. A directional reactive-ion etch isused to remove the unprotected areas. Finally, a photo-resist strip isutilized to remove the photolithographic mask.

In an alternate embodiment of the invention, forming the pillar alsoincludes depositing a first layer between the phase change material 604and the bottom electrode 104. In this particular embodiment of theinvention, the first layer is oxidized such that a portion of the firstlayer is thermally insulating and a portion of the first layer iselectrically conducting. In one particular embodiment of the invention,the first layer is comprised of Tantalum (Ta).

In another embodiment of the invention, forming the pillar also includesdepositing the first layer between the phase change material 604 and thebottom electrode 104, and depositing a second layer between the topelectrode 606 and the phase change material 604. In this particularembodiment of the invention, the first layer and second layer areoxidized such that a portion of the first layer and a portion of thesecond layer are thermally insulating and a portion of the first layerand a portion of the second layer are electrically insulating. In oneparticular embodiment of the invention, the first layer and the secondlayer are comprised of Tantalum.

Now turning to FIG. 7, a film of thermally insulating material 702 isdeposited along at least one sidewall of the pillar and over a portionof a top surface of the top electrode. In one particular embodiment ofthe invention, the film of thermally insulating material 702 iscomprised of Tantalum Oxide. As stated above, the film of thermallyinsulating material 702 helps isolate the heat created by ohmic heatingto the materials comprising the pillar.

In FIG. 8, a dielectric layer 802 is deposited above the film ofthermally insulating material. In one embodiment of the invention, thedielectric layer 802 is comprised of Silicon Dioxide. Those skilled inthe art will recognize that a variety of processes may be utilized todeposit the dielectric layer 802, such as CVD and PECVD. Additionally, adry etch, a wet etch, and a chemical mechanical polish (CMP) may be usedto finish the surface.

In FIG. 9, an alternate embodiment of the invention is illustrated. Inthis particular embodiment, the film of thermally insulating material702 is deposited along at least one sidewall of the pillar. The pillar,in this particular embodiment, includes the first layer and the secondlayer (as mentioned above). As illustrated, a portion of the first layerand a portion of the second layer are thermally insulating and match thefilm of thermally insulating material 702. Additionally, a portion ofthe first layer and a portion of the second layer are electricallyconducting 902. The electrically conducting portions 902 of the firstlayer and the second layer allow current to flow from the bottomelectrode 104, to the phase change material, to the top electrode. Thefilm of thermally insulating material 702 helps isolate the heat to thephase change material and the top electrode.

Having described preferred embodiments for the method for forming amemory cell structure (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope and spirit of theinvention as outlined by the appended claims. Having thus describedaspects of the invention, with the details and particularity required bythe patent laws, what is claimed and desired protected by Letters Patentis set forth in the appended claims.

What is claimed is:
 1. A memory cell structure, the memory cellstructure comprising: a bottom electrode comprised of electricallyconducting material; phase change material disposed above the bottomelectrode; a layer of thermally insulating material disposed, at leastpartially, between the bottom electrode and the phase change material;and a top electrode comprised of electrically conducting material; andwherein the thermally insulating material is comprised of TantalumOxide.
 2. The memory cell structure of claim 1, including a layer ofthermally insulating material disposed, at least partially, between thephase change material and the top electrode.
 3. A memory cell structure,the memory cell structure comprising: a bottom electrode comprised ofelectrically conducting material; phase change material disposed abovethe bottom electrode; a layer of thermally insulating material inphysical contact with both the bottom electrode and the phase changematerial; and a top electrode comprised of electrically conductingmaterial; and wherein the layer of thermally insulating material isdisposed, at least partially, between the bottom electrode and the phasechange material.